Under the 3GPP standards, a NodeB (or an eNB in LTE) is the base station via which mobile devices connect to the core network. Recently the 3GPP standards body has adopted an official architecture and started work on a new standard for home base stations (HNB). Where the home base station is operating in accordance with the (Long Term Evolution) LTE standards, the HNB is sometimes referred to as a HeNB. A similar architecture will also be applied in the WiMAX network. In this case, the home base station is commonly referred to as a femto cell. For simplicity, the present application will use the term HNB to refer to any such home base station and will use the term NodeB generically to refer to other base stations (such as the base station for the macro cell in which a HNB operates). The HNB will provide radio coverage (for example, 3G/4G/WiMAX) within the home and will connect to the core network via a suitable public network (for example via an ADSL link to the Internet) and in the case of the 3GPP standards, via an optional HNB gateway (HNB-GW) which typically will aggregate traffic from several HNBs.
Typically, when initiating a call, user equipment (UE) such as a mobile telephone (MT) or other mobile communications device (MCD) will register either with a HNB or a NodeB based on, for example, the ability of that HNB/NodeB to support the call relative to other HNBs/NodeBs in the vicinity. Once the call is initiated, it sometimes becomes necessary to relocate the call to another HNB/NodeB due to a change in the relative ability of the original HNB/NodeB to continue to support the call, for example as a result of a change in the location of the UE or other factors which affect the quality of service offered by the original HNB/NodeB. In the case of HNB to HNB relocation, the relocation may also require a change in the serving HNB-GW (Inter-HNB-GW relocation). When a UE is constantly moving (for example when it is located in a moving vehicle) relocation can occur several times during a single call.
The HNB may be configured to operate using one of a plurality of access modes, namely: ‘closed’ in which the HNB operates as a closed subscriber group (CSG) cell; ‘hybrid’ in which the HNB operates as a CSG cell where at the same time, non-CSG members are allowed access (for example, to allow preferential treatment of CSG members); and ‘open’ in which the HNB operates as a normal (non-CSG) cell.
During the initial UE registration procedure access control is performed by the HNB-GW in dependence on the CSG capability of the UE. In the case of non-CSG equipment (e.g. pre-release 8 user equipment), according to the current version of the relevant standard (3GPP TS 25.467 V8.0.0), access control during registration is mandatorily perfointed at the HNB-GW, and optionally at the HNB. In the case of CSG equipment (e.g. release 8 user equipment), the access control mechanism at the HNB-GW always accepts UE registration and subsequent access control is performed by the core network.
In the case of an inter-HNB-GW relocation (e.g. of a CSG UE), after the relocation the new source HNB-GW has no means to determine whether or not access control should be performed for a subsequent relocation. Accordingly, access control may not be performed correctly.
Furthermore, if access control is performed by the new source HNB-GW (for example, as for a non-CSG UE) during relocation, and it is established that the target HNB is configured to operate in a closed access mode, the HNB-GW will reject access, potentially resulting in the call being dropped. This is very undesirable if the original call is an emergency call.
It is currently a requirement in 3GPP for relocation requests (e.g. the RANAP RELOCATION REQUEST) to include an International Mobile Subscriber Identity (IMSI) (e.g. in the ‘Permanent NAS UE Identity’ information element) for the UE for which relocation is requested. However, a UE registered with a HNB may identify itself using a choice of several identities including an IMSI, a Temporary Mobile Subscriber Identity (TMSI), a Packet Temporary Mobile Subscriber Identity (P-TMSI), an International Mobile Equipment Identity (IMEI), etc. Accordingly, after relocation there is a danger that the UE identity as linked to the original context may be lost.
Catering for NodeB to HNB (‘macro-to-femto’) relocation and HNB to HNB (femto-to-femto) relocation therefore presents a number of challenges, in particular for access control, and especially in the case of Inter-HNB-GW relocation.